This invention relates to a control system for a batch-type pyrolysis device of the type used for volatilizing and burning organic material from a metal part to which the organic material is bonded. Incineration occurs in a zone adjacent to the device's main chamber in which the material is volatilized. Such an incineration zone is typically provided by an afterburner chamber in which an afterburner, positioned downstream of the device's main chamber in which pyrolysis occurs, burns the volatilized organic material (referred to herein as "vapor"). The remaining metal part is reclaimed for reuse because the cost of reclamation is less than that of making the metal part anew. Such reclamation by pyrolysis has evolved into a subindustry of considerable economic significance not only because pyrolysis is cost-effective, but also because incineration of the vapor of polymeric materials which are not economically recyclable, conveniently and beneficially disposes of them.
A highly successful control system for a pyrolysis furnace in which incineration occurs in the main chamber and also downstream of the afterburner is provided in our U.S. Pat. No. 4,649,834. This system uses a single water spray system controlled by a first thermocouple (TC) in the main chamber (main chamber TC), a second thermocouple in the stack (stack TC) which controls only on/off or attenuated operation of the main burner and a third thermocouple (throat TC) in the vent passage ("throat") connecting the main chamber to the afterburner chamber. The effectiveness of this control system, in large measure, derives from the difference in temperatures sensed by the first and third thermocouples.
This invention is specifically directed to burning relatively large loads of metal parts combined with silicone-free polymers ("burnables") which are to be incinerated smokelessly in a relatively small main chamber, that is, with a relatively high ratio of load (lb)/volume (ft.sup.3), referred to as the load/volume ratio. Such loads contain from 0.1 lb of burnables per lb of metal, to 2 lb burnables/lb metal, and are referred to as "high-polymer" loads in contrast to conventional loads which contain less than 0.1 lb burnables/lb of metal.
The term "pyrolysis oven" has been used in the art to indicate that there is no incineration of organic material on the metal parts within the oven's main chamber. The material is simply volatilized (or vaporized) without being burned in the oven's main chamber. The vapors are then burned in the afterburner chamber, but not before they have exercised the opportunity to plug water spray nozzles used to keep the volatilization of burnables in the main chamber under control. Such operation of a "pyrolysis oven", where there is no fire in the main chamber, is supposed to clearly distinguish its function, from that of a "pyrolysis furnace" in which there is. Nevertheless, the terms are often misused or interchanged, particularly in relation to devices using an afterburner in an afterburner chamber of the furnace, with no thought given as to the significance of where the fire is maintained.
It was found that with high-polymer loads loads with the above-specified burnables content, the rise of temperature in the initial portion of the burn cycle was often uncontrollable, resulting in dense smoke and excessive temperatures in the main chamber. This occurred even when the furnace is constructed with a "vent number" greater than 0.003/ft found to be critical for normal operation. The vent number is computed by dividing the area of the vent (throat, ft.sup.2) by the volume of the main chamber (ft.sup.3).
In our copending patent application Ser. No. 881,953 filed July 3, 1986, we provide a pyrolysis furnace which generates a smokeless discharge into the atmosphere under normal conditions of commercial operation--a characteristic of operation not duplicated in any prior art device we know of. By "discharge" we refer to combustion products issuing from the furnace's stack, and by "smokeless" we refer to the discharge being substantially clear to the naked eye, that is, permeable to light in the visible wavelength range.
The problem with our above-identified systems is that, to ensure a high degree of safety of operation, and to minimize the danger of an explosion, it was necessary to use plural thermocouples (TCs). The redundant safety provided by the additional TCs was far more essential than tests originally indicated, the single TC in the throat being simply unable to provide as large a margin of safety as is desirable. The location (in the throat) of the essential TC was arrived at from earlier experimentation which resulted in our use of a combination of three thermocouples (our '834 patent), but safe operation with a single TC in the throat, required operation of the furnace with more care than is likely to be provided by unskilled operators. Particularly because the location of one TC in the throat was also maintained in a three-thermocouple system used in our copending application Ser. No. 881,953, it appeared unlikely that both, close control over the temperature of the parts as well as a high enough degree of safety might be predicated upon repositioning the throat TC.
Moreover, the problem of maintaining the calibration of plural TCs, not to mention the cost of installing them, provided the incentive to dispense with all but one of the TCs. But it was difficult to believe that re-positioning the throat TC could make the big difference--after all, we had reason to believe that we had already placed the TC in the most advantageous location, namely in the throat. It was therefore with considerable surprise that we learned from measurements we made, that the most sensitive location for a single TC was in a zone outside, but proximately disposed relative to the throat, in the uppermost part of the main chamber, no more than about 1 foot from the upper edge of the throat, regardless of the size of the main chamber, provided other explosion-defeating design criteria were preserved. This zone is referred to herein as the "critical sensitive zone" or "CSZ" for brevity. These other design criteria are more fully described in our '834 patent and copending application Ser. No. 881,953, both the disclosures of which are incorporated by reference thereto as if fully set forth herein.
The vapor to be incinerated is generated when mounting means for engines and electric motors (collectively referred to as "motor mounts"), and similar steel parts bonded to rubber; or, copper-containing electrical parts such as armatures, stators, transformers and the like; or, painted ferrous or non-ferrous steel parts; or, metallic bodies of arbitrary shape which are coated with, or bonded to polymeric materials (referred to herein as "polymer-bonded metal parts"), are to be pyrolized in a pyrolysis furnace.
Polymeric materials to be disassociated from metal parts are such materials as are commonly bonded to a metal substrate or matrix and include natural and synthetic elastomers; for example, natural rubber and synthetic rubber which are polymers of dienes; silicones which are polymers of siloxanes and the like; and, natural and synthetic resinous materials including natural shellac and synthetic plastics such as phenolics and acrylics, particularly paints. The difficulty of incinerating the materials smokelessly varies; silicones do not burn smokelessly, but silicone-free rubbers and paints can now be reliably and economically incinerated, and smokelessly.
The foregoing polymeric materials are to be separated from the metal matrix to which they are bonded without melting the metal, and preferably, in most instances, without causing warpage or other undesirable deformation of residual metal matrix. It is self-evident that such separation may be effected by directly incinerating the polymeric materials, as is typically done in an incinerator for waste, but it is equally self-evident that the requirement of incineration without damaging the metal parts will not be met. Of course, damage to the parts can be minimized if only a few parts are incinerated together, but this method is undesirable because it does not lend itself to reclaiming a large enough mass of parts to be economical.
The desirability of a smokeless discharge from the stack of a pyrolysis furnace cannot be overemphasized. It is common practice to operate such a furnace during the day in such a manner that the smoky discharge is not too objectionable, reserving such operation for darkness. More responsible operators provide plural afterburners in series to make sure that as complete combustion as possible is obtained. The seriousness of the problem is such that even in a drying furnace where a relatively small amount of contaminating oil is being burned, plural burners are used, as disclosed in U.S. Pat. Nos. 3,767,179 and 3,839,086 to Larson.
Where the weight ratio (weight of burnables to be burned): (weight of metal) is relatively high, that is in the range from 0.1:1 to 2:1, a manufacturer of a prior art furnace advises against burning such loads. Attempts to burn even a small load result not only in the discharge of a highly noticeable stack gas, but also in the severe fouling of the furnace's main chamber, the controls, and, most important, of the water nozzles upon which the safe operation of the furnace is critically dependent.
An attempt to deal with the problem of fouling water spray nozzles is found in U.S. Pat. No. 4,557,203 to Mainord who uses a first sensor in the stack downstream of the afterburner to actuate a first set of nozzles; and a second sensor in the main chamber to actuate a second set of nozzles.
Prior to our invention (in the 881,953 application) we did not realize that the sensitivity of the throat thermocouple is such that, a controlled rate at which the temperature of the load is raised ("ramped") can control a burn so effectively as to provide a smokeless stack even when burning a load of high-polymer parts. And most important, that the entire burn cycle may be controlled with the throat TC, so that the main chamber TC and the stack TC are used to provide only redundant safety of operation. Thus we were also unaware that there existed a critical location for the main chamber TC which not only could replace the function of the throat TC, but would do so with such safety that the other thermocouples couldbe eliminated.
This invention is specifically directed to a pyrolysis furnace with a single thermocouple, located in the CSZ, and peculiarly sensitive to the temperature generated by a fire sustained in the furnace's main chamber. The temperature is ramped to preselected progressively higher setpoints with optional intervening soak intervals, after which the temperature is maintained constant during a final load-cleaning burn (referred to as the "final soak period"). The surprising result is that there is essentially no visible smoke issuing from the stack, and no runaway increase of temperature.
A charge of metal parts on a cart is charged to the main chamber, the charge is brought up to ignition temperature at a predetermined rate which is controlled by a programmable control means, ignited, and the fire sustained under controlled "ramp and soak" conditions until the charge is burned out.
It is known that heating of the metal parts to 700.degree.-800.degree. F. in an enclosure with limited air intake will char or degrade all known combustible contaminants without ignition if the percentage of contaminants is less than about 2% by weight ("wt") of the parts. However, we are concerned with igniting much higher amounts of combustibles in the range from about 10% by wt of the load in the charge to about twice the weight of the load, or even more, and it is critical that the ignition result in an essentialy smokeless stack.
It is unnecessary to point out that, when operating under near-explosive conditions and a very small misstep can set off an explosion, a smokelesss stack may be an exiguous consideration. But any control system which provides a smokeless stack, yet prevents such an explosion from being set off, acquires great merit. In other words, a smokeless furnace must be operated with no sacrifice of safety. Our invention does so.
A reclamation oven with a control system for preventing fires and explosions and thus controlling excess temperature within it, is disclosed in U.S. Pat. No. 4,270,898 to Kelly. The fire and explosion control method senses a fire situation before it occurs, and keeps the fire from happening by instituting a timely extinguishing system. A thermocouple is installed in the exhaust, downstream from the afterburner, and when the temperature exceeds a preset temperature, a signal from the thermocouple actuates an automatic valve assembly to open it and spray water onto the too-hot parts in the main chamber. When the parts cool sufficiently, the valve assembly closes. The system prevents fires and explosions and thus controls excess temperatures. The main burner is not shut off when the water spray comes on, though the main burner goes off when the oven reaches the set-point temperature, nor is the average temperature above the metal parts in the oven's main chamber (referred to as the "ambient temperature" in the main chamber) monitored. It is evident that the possibility that a temperature monitored at some location in the main chamber might be a critical factor in the control of the operation of the furnace, escaped the patentee. Moreover, this prior art system, in which a fire in the main chamber is prevented, is wholly ineffective to minimize the smoke issuing from the stack, and as Mainord states, is responsible for plugging water nozzles. It is quite unlike our system in which a fire is maintained under conditions imposed by alternately ramping temperature, then maintaining it constant ("soaking").
Another system relating to incineration of unwanted organic material such as oil associated with metal parts, particularly scrap or swarf, is disclosed in U.S. Pat. No. 3,705,711 to Seelandt et al. Only as much air and fuel as is required to fuel the main burner, is burned to minimize oxidation of the metal parts and to minimize the risk of explosion. It is evident that such conditions of operation are calculated to generate more smoke because of incomplete combustion, not minimize the smoke generated. Control is provided by limiting the amount of combustion air to the main chamber when a preset pressure is exceeded. It is suggested that the temperature within the drum may first be lowered by throttling back the main oil burner or by stopping the feeding of metal scrap into the dryer drum, though it appears that control of the temperature is only a secondary consideration, there being no indication that the degree of control might be affected by where the temperature was measured. When the main burner output is reduced to its lower limit and the temperature within the drum is still too high, a water spray may be actuated. Should the spray be insufficient to lower the temperature, the feeding of the scrap into the drum is reduced or stopped. The problem is that the time period required for these operations is much longer than that permitted by conditions under which an explosion occurs because of ignition of the built-up vapor. As a result, such a system is wholly unsatisfactory under the conditions of operation of a pyrolysis furnace.
The control system of our invention allows the safe and smokeless burn of a high-polymer load by controlling a single stage of the burn cycle, namely the ramping stage. Control of the temperature in the CSZ, with a single TC and PC to track the ramp and soak profile, and actuate an intermittent water spray as required, is the only essential requirement of our single-stage system.
No prior art control system for a pyrolysis furnace recognized the importance of controlling temperature in a CSZ, for safe operation which produced a smokeless discharge.